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9-Jun-2008 20:00 Eastern US Time

Behind the mask

A busy street corner. A crowded cocktail party. A rainforest at twilight. All places where sounds from many sources mingle. In doing so they create what the authors of a paper published today call "a highly convoluted and complex acoustic environment".

Yet in these noisy places we humans can still hear what's important to us, and so can other animals.

Warning cries, mating calls, the whispered words of a hot date in a noisy restaurant - these can all be heard above the "background cacophony". Now Alexander Gutschalk (Heidelberg University), Christophe Micheyl and Andrew J. Oxenham (both University of Minnesota) have discovered where in the brain this happens.

From ear to brain

The part of the brain known as the auditory cortex is a complex piece of kit that artificial intelligence researchers would love to have in their machines. It can do clever things that are hard to reproduce with technology and were, until recently, almost invisible to science. Cutting brains open can reveal a lot about their structure. It can't tell us what the different parts are doing, because by then they are not doing much.

But a variety of methods have been developed in recent years to probe the living brain. For their experiments Gutschalk and colleagues used a technique called magnetoencephalography. This detects tiny magnetic fields produced by electrical activity in the brain. Its big selling point is that it can separate signals that are close to each other in time. It has high temporal resolution. So magnetoencephalography can "track the dynamic aspects of functional processes in real time".

This is just what the scientists wanted, as they tried to figure out how some sounds make it into our consciousness while others, often just as loud, get masked.

From brain to hearing

Many examples of masking can be explained by the way sounds are processed in the inner ear, say the authors. "The background or masking sound produces a pattern of excitation in the cochlea that either swamps or suppresses the activity due to the target sound."

This is called energetic masking. But it only works when the sounds are regular and predictable. So the most interesting kind of masking - "masking under conditions of uncertainty and timbral similarity" - can't be explained so easily.

To study this cocktail party kind of masking - informational masking - the research team performed a set of experiments on 33 people with normal hearing. Their aim was to "isolate brain responses that correlate with conscious auditory perception". They were looking, in other words, for brain signals that happen when, and only when, a sound makes it into our awareness.

Signals in the brain

The volunteer subjects were asked, while wired up to the magnetoencephalography instrument, to say when they could hear a particular tone against a noisy background. The tones they were trying to hear - the target tones - were regularly repeated and unchanging in frequency. The background noise, on the other hand, was a set of tones that occurred at random times with random frequencies.

The researchers found two distinct signals in the cortex due to the target tones. The first appeared soon after the tones began to sound, and whether the subject was aware of the tone or not. The second began after 50 milliseconds or so - and it only appeared if the subject was aware of the target tone.

Hear this

What this means is that these tones are always getting past the ear and into the brain - which they wouldn't with the simpler energetic masking. But once in the auditory cortex something fascinating and mysterious occurs, which through this type of experiment is beginning to yield answers to science. The brain somehow chooses which parts of the world around us are selected to become part of our consciousness.

"We propose that this component specifically reflects conscious sound perception," write the authors. "In contrast, earlier responses in the auditory cortex were evoked by both detected and undetected target tones.

"These results suggest that conscious sound perception emerges from within the auditory cortex."

Teachers wanting to learn more about magnetoencephalography, and how it can be combined with electroencephalography to get detailed knowledge of what's going on in the brain can try this pdf colour booklet

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